The central interest of this laboratory concerns the mechanisms involved in the release of vasoactive compounds as a consequence of an immune event at or near a cell surface. An in vitro system has been constructed for estimates of histamine release from human leukocytes as a means of studying ragweed sensitivity in humans. The reaction system comprises washed leukocytes, and a buffer diluent containing calcium, magnesium, and a highly purified protein extract of ragweed pollen. The release of histamine into the fluid phase of the reaction mixture entails several reaction steps, including antigen-antibody interaction at the cell surface and others, as yet unknown, requiring calcium and magnesium. The release of histamine from peripheral leukocytes does not necessarily imply irreparable cell injury. Passive sensitization of cells from nonallergic donors can be achieved. A marked individuality is apparent in the ability of cells from different individuals to bind sufficient antibody for the release of histamine upon interaction with ragweed antigen. The presence of this vasoactive amine in the extracellular reaction medium can be mediated by antigens other than pollen extracts indicating that human leukocytes normally carry bound immunoglobulins on their surfaces. The reaction system offers a practical and in vitro means for assay of pollen antigens, and possibly of skin-sensitizing antibodies.